1. Technical Field
The invention relates to frequency conversion, for example for cable television applications. More particularly, the invention relates to a common carrier, multiple output RF upconverter.
2. Description of the Prior Art
FIG. 1 is a block schematic diagram showing a conventional multiple carrier, multiple output RF upconverter. A typical frequency agile RF upconverter designed for cable television (CATV) applications uses a dual frequency conversion process in which a first local oscillator (LO) 10 typically operates at a fixed frequency in the range from 900 MHz to 1100 MHz and a second LO 12 operates in the range from 1 to 2 GHz, depending on the desired output frequency range.
The RF upconversion process actually incorporates an upconversion step and a downconversion step to attain the desired frequency range and elimination of in Band spurious response. The first LO, in conjunction with a first mixer 13, upconverts an incoming 44 MHz intermediate frequency (IF) signal 14 to a new, higher IF frequency, typically in the range from 950 MHz to 1150 MHz. This signal is then bandpass filtered 16 to remove the unwanted carrier generated by the mixing process. The filtered high/IF is then sent on to the next mixing stage 18 where it is downconverted to a frequency in the CATV spectrum ranging from 50 to 860 MHz.
In a frequency agile upconverter, the second LO, which is comprised of a wide bandwidth voltage controlled oscillator (VCO), provides the frequency setting agility to span the 50 to 860 MHz frequency range mentioned above.
Both the first and second LOs require high stability frequency synthesizers to meet phase noise, frequency stability, and frequency setability requirements. A substantial portion of the circuitry in the typical upconverter is dedicated to the support and operation of the first and second LO synthesizers.
For specific applications in CATV that require a common frequency but with multiple RF outputs, such as broadcasting node specific video content or data over multiple nodes using a fixed channel frequency, the current practice is to rely on an individual RF modulator/upconverter for each node, e.g. Chan#1, Chan#, Chan#3, . . . , Chan#N. Using individual modulators/upconverters can consume a large amount of equipment rack space. For most CATV headends, space is scarce.
It would be advantageous to provide a mechanism for substantially reducing the board space and component count normally required to perform the modulation/upconversion function.
The invention provides a mechanism for implementing a common carrier, multiple output RF upconverter. The preferred embodiment of the invention comprises a method and apparatus for distributing the LO signals for both the first and second LO functions to several upconverter sections simultaneously. This technique eliminates the frequency synthesizers and VCOs and associated circuitry typically required for each individual upconverter section, thereby substantially reducing the board space and component count required to perform this function. By providing a distributed LO, only two LO synthesizers and VCO""s are required per board, where a board might support anywhere from two to twelve or more upconverters.